One such conventional hardness tester, which was proposed by the inventor of the present invention, is disclosed in the Publication of the Japanese Utility Model Registration No. 3,028,586. The configuration and operation of the hardness tester is as follows.
Referring to FIGS. 9 and 10, the hardness tester includes a first unit 53 and a second unit 56. The first unit 53 has a main leg 52, which is urged downward by a compression spring 51 and moves upward against the compression spring 51 when pressed against an object part of a living body (e.g. a living tissue), and a display unit 62, which digitally displays an amount of displacement of the main leg 52. The second unit 56 has a pair of side legs 55, 55 provided on both sides of the main leg 52. The side legs 55, 55 are urged downward by tension springs 54, 54 weaker than the compression spring 51 and move upward against the tension springs 54, 54 when pressed against the object part. The second unit 56 further includes a retaining mechanism 50 for retaining the amount of displacement displayed by the display unit 62 at the moment the side legs 55, 55 have moved by a predetermined amount.
In the retaining mechanism 50, a module plate 41 with a rubber plate 42 fixed on its back is mounted on the top of the main leg 52, and is guided by a pair of guide rails 63 to slide vertically. A slider 58 which slides vertically on a pair of guide poles 57, 57 is mounted on the top of the pair of side legs 55, 55. A notched step part 43 having a reversed-U shape is formed in the slider 58 in opposition to the rubber plate 42. The notched step part 43 has a notch 44 formed at the center of the slider 58 and a pair of steps 45 formed on both sides of the notch 44 at the back of the slider 58, where each step 45 has a sloped face 59 at its lower end. A stopper pin 48 having a serrated face 46 at its front end and a cross pin 47 in its body, is urged by a compression spring 49 toward the rubber plate 42 so that the front end is posed in the notch 44 of the notched step part 43 and the cross pin 47 contacts with the step 45 at both sides.
In the above-described hardness tester, when the main leg 52 and the two side legs 55, 55 are pressed against the object part, the legs move upward against the springs. Here, the movement of the slider 58 is larger than that of the module plate 41 since the tension springs 54, 54 for the side legs 55, 55 are weaker than the compression spring 51 for the main leg 52.
When the slider 58 ascends to a preset height, the cross pin 47 of the stopper pin 48 is displaced off the steps 45 of the notched step part 43. At this moment, the stopper pin 48 is activated by the compression spring 49 to plunge forward, as shown in FIG. 10, so that the pin 48 presses the rubber plate 42 at the back of the module plate 41 with its front end. Thus, the module plate 41 is held under compression against a scale 60.
In the above state, the module plate 41 is securely fixed at the position since the serrated face 46 provided at the front end of the stopper pin 48 embeds itself into the rubber plate 42. The hardness of the object part in contact with the main leg 52 is digitally displayed by the display unit 62. In this state, the stopper pin 48 is pressed against the rubber plate 42 not only by the compression spring 49 but also by the tension springs 54, 54 since the cross pin 47 is pressed forward by the sloped faces 59 of the lower front ends of the steps 45 activated by the tension springs 54, 54.
When the knob 64 at the back of the casing is pulled against the compression spring 49, the serrated face 46 at the front end of the stopper pin 48 releases the rubber plate 42, so that the main leg 52 is activated by the compression spring 51 to return to its original position automatically and the digital display of the display unit 62 is also reset.
The above-described hardness tester of the Japanese Utility Model Registration No. 3,028,586, however, is accompanied by some problems as follow.
(1) Since the cross pin 47 of the stopper pin 48 is pressed against the slider 58 by the compression spring 49 (as shown in FIG. 10), the slider 58 encounters frictional resistance during the ascent so that the stress in the compression spring 51 for the main leg 52 is larger than theoretically estimated. Thus, the hardness of the object part cannot be measured accurately.
(2) After the hardness tester has been in use for a long time, the effective grip by the serrated face 46 on the surface of the rubber plate 42 by the stopper pin 48 becomes more difficult due to frequent abrasion. In this case, the main leg 52 is pushed back by the compression spring 51 even when the module plate 41 is clamped between the scale 60 and the stopper pin 48.
(3) The pressure mechanism for stopping the module plate 41 utilizes the stress of the compression spring 49. This requires such a large number of parts that errors may easily occur in assembling, which lowers the reliability of the measurement by the hardness tester. Furthermore, the hardness tester including many parts is not easy to operate.
(4) The main leg 52 and the side legs 55, 55 are each separated by a considerably large distance. Therefore, when the hardness of an uneven part of a living body or a part with a small radius of curvature is measured, a measurement error is inevitable.
Publication No. S51-84684 of the Japanese Unexamined Patent Application discloses another hardness tester including a cylindrical guard ring which is located by sliding onto the side of an indenting rod provided in the center of the lower part of the hardness tester. The configuration of this hardness tester, however, is very complicated since it utilizes a strain gauge, hall element, photoelectric element, or differential transformer to build up a system for detecting the displacement of the indenting rod electrically.
In addition, any of the conventional hardness testers has only one detection rod (penetrator, indentor, feeler, etc.). Therefore, during a measurement using the conventional hardness tester, the rod may easily tilt, resulting in an incorrect measurement result.
In measuring the hardness of a part of a living body, particularly, the measurement result easily changes even by a small displacement of the measurement point or by a small tilt of the detection rod. Therefore, when a hardness tester with a single detection rod is used to measure the hardness of an object point, it is necessary to measure the hardness of a plurality of points around the object point and take the average of the hardness at the points to obtain a reliable measurement result.